1. Field of Invention
This invention relates to low lift pallet trucks, specifically to those used to transport loads on double-faced pallets.
2. Description of Prior Art
Prior construction low lift pallet trucks are commonly used in retail stores, warehouses and factories for moving palletized loads. In order to move loads on double-faced pallets, it is necessary that the truck's forks enter the pallet. FIG. 1 shows a typical prior construction truck and FIG. 5 shows a typical double-faced pallet.
During fork entry these trucks must climb onto and roll along the pallet bottom boards. Once fully positioned, the fork lift frame is elevated as the forks wheels pass to a position over the void in the pallet bottom boards. The pallet is then elevated clear of the floor and truck travel initiated.
At arrival at its destination, the fork frame is lowered and the pallet deposited. Exit of the forks again requires the climb and roll along the pallet bottom boards.
Prior construction trucks have proven inconsistant in these entry and exit operations. Various devices such as skids, rollers, and discs are mounted to the fork underside fore and aft of the fork wheels. However variations in pallet flatness, bottom board thickness, and bottom board warpage as well as floor slickness often cause pallet movement with the truck. Additionally, misalignment of the truck's fork tips at entry, excessive fork wheel loadings, contamination of the afore-mentioned entry/exit devices and fork wheel wear contribute to their unreliability. Entry into and exit from unloaded and lightly loaded pallets is particularly bothersome.
The more commonly known sit-down counter-balance type lift trucks for instance do not have such pallet entry and exit problems. Their forks extend out entirely beyond their fork end load bearing wheels. Low lift pallet trucks, despite the serious operational problems described have several advantages over other fork trucks. Much lower initial cost and weight, simplicity of operation, and lower required operating space all account for their popularity. To improve the reliability of low lift pallet trucks it is important to improve their entry and exit performance by reducing horizontal reactive forces on pallets used with these trucks.
U.S. Pat. No. 4,615,533 to Sewell (1986) describes the addition of a complicated hydraulic system to tilt back a fully lowered truck before entry by means of one or two hydraulic cylinders, and small rollers. This system requires additional driver operations, judgement, and time. The addition of one or two hydraulic cylinders, extension springs, slide assemblies, a hydraulic electric valve, electric switches and wiring, and their mountings and cover, all mounted outside of the truck body, make this method and apparatus unduly expensive and burdensome in operation.
Since all descriptions and claims in the Sewell patent are for a pressurized hydraulic system, I would add the likely increased occurrence of oil leakage at the many added oil connections to be another disadvantage of his system.
In the patent searches I made for low lift pallet trucks patents granted from 1941 thru April of 1993, I found only one patent of the lower linkage using rotating lower links. This is U.S. Pat. No. 2,359,493 to Schreck (1944) which recites the lower link, rod assembly (but of one rigid length), and fork wheel lever and without elevation of the fork wheels clear of the floor.